This invention relates to a process for extracting oils and fats from vegetable raw materials by initially reducing the size of these raw materials, conditioning and subjecting the raw materials to an extraction by means of solvents; the recovery proper of oils and fats and solvent from the extract is then completed.
Oil containing seeds with a high fat content are generally extracted in two steps. First, a part of the fat or oil is removed by pressing the material and secondly, a quantity of fat or oil is removed by means of extraction by a solvent. When seeds with a low fat content are to be treated, pressing them has little effect, so that an extraction is then generally only carried out by means of a solvent.
An extraction process was only thought to be effective so far when the size of the raw material to be extracted was reduced. This was done by rolling the raw material until only thin flakes were left. Such a rolling treatment had not only to be carried out with great care and by means of smooth rollers, but it also consumed much energy. Moreover, many irregularities occur during the rolling operation, particularly during the extraction treatment, which can give rise to unsurmountable difficulties.
This process is moreover expensive, as the rollers consume much energy and require much upkeep.
In order to enhance the extractability of the raw material, the same is usually, after the rolling operation, submitted to a thermal treatment up to 100.degree. C. which is maintained for some time. It is sometimes necessary to maintain a high humidity during this heating period, which can be realized by heating with open steam, and by adding water, if required.
After the heating period, the material is dried until the desired extraction humidity is obtained.
The aforementioned heat treatment is mostly carried out in vertical cylindrical vessels which are partially double-walled, so that the material is heated indirectly by means of steam. The vessels are divided into a number of compartments by double-walled partitions. By means of a stirrer and a valve system, the raw material to be treated is passed through them from top to bottom.
The entire residence time of the material in these vertical cylindrical vessels ranges from about three quarters of an hour to 2 hours.
A treatment in these cylindrical vessels is, however, very inconvenient, as the raw material is hampered hereby. Owing to a bad heat transfer, high temperatures of the walls of these cylindrical vessels are required. Moreover, ineffective mixing of the material and an excessive residence in the vessels inevitably cause a part of the raw material to be exposed to excessive heating, so that decomposition products are produced which impair the quality of the oil.
Owing to the high temperatures, moreover, a denaturation of the proteins in the solid material is produced which gives rise to a decrease in value of the extracted meal.
After the thermal treatment in the vertical cylindrical vessels, extraction by means of a solvent takes place, whereby two flows are produced, viz. a flow of solvent mixed with oil, and a second flow of solid material mixed with a solvent.
The flow of solvent mixed with oil is separated into oil and solvent by means of an evaporator and a stripping column; the second flow of solid material mixed with solvent is passed to a cylindrical vessel where the solvent is removed. The remainder of the solid material is finally dried and used as fodder or raw material for the recovery of proteins.
During the latter treatment excessive temperatures are also inevitable, which decreases the value of the proteins owing to denaturation.